1. Field of the Invention
The present invention relates to an FM tuner for receiving frequency-modulated (FM) signals.
2. Description of the Related Art
With FM signals, the frequency of a carrier wave is changed on the basis of an audio signal or the like; therefore, a frequency bandwidth that is wider than for, e.g., AM signals, is required when they are transmitted. Accordingly, when an FM tuner receives a desired transmission signal, the tuner is readily subjected to interference from other signals transmitted at frequencies that are close to the frequency of the desired transmission signal (adjacent interference). Adjacent interference is capable of adversely affecting the quality of the audio signals detected from the reception signals. Specifically, adjacent interference occurs in a case where another station exists on a frequency that is close to that of the station the listener desires. The quality of the audio output reproduced by the FM tuner will also deteriorate in a multipath reception state, in which waves directly received from a radio broadcasting station are received along with waves reflected by buildings or other objects along the transmission path (multipath interference state). Adjacent interference and multipath interference are undesirable in radio data systems (RDS) wherein text data or the like is superimposed on an FM radio broadcast signal and transmitted.
FIG. 4 is a block diagram showing a configuration of a conventional FM tuner. An RF (radio frequency) signal received by an antenna 2 is frequency-converted to a first intermediate signal SIF1 having a first intermediate frequency (IF) fIF1, the SIF1 is frequency-converted to a second intermediate signal SIF2 having a second intermediate frequency fIF2, and the SIF2 is input to an IFBPF 4. The IFBPF 4 is a bandpass filter having a frequency fIF2 as a center frequency. A bandwidth WF of the filter can vary within a range of, e.g., about 40 kHz to about 220 kHz.
An FM signal that has passed through the IFBPF 4 is inputted to an FM detection circuit 8 via a limiter amp 6. The FM detection circuit 8 FM-detects an output signal of the limiter amp 6, and outputs a detection signal SDET.
An S-meter circuit 10 receives the first intermediate signal SIF1, generates a signal SM-AC that corresponds to an amplitude variation component included in the inputted signal, and then smoothes the variation component using a low-pass filter (LPF) and generates a reception field strength signal SM-DC. Included in the variation component signal SM-AC is a component that corresponds to adjacent interference and multipath interference.
A high-pass filter (HPF) 12, a detection circuit 14, and a comparator 16 are provided as circuits for detecting the presence or absence of adjacent interference and multipath interference on the basis of SM-AC. The HPF 12 is capable of switching the cut-off frequency fc according to whether one or the other of a frequency band component corresponding to adjacent interference or a frequency band component corresponding to multipath interference is extracted from SM-AC. Using a CR circuit, the detection circuit 14 detects the high-frequency component having passed through the HPF 12, and performs a conversion to DC voltage VSQ. The comparator 16 compares an output level VSQ of the detection circuit 14 with a reference voltage Vref1 set to a predetermined threshold value. For example, if VSQ>Vref1, a predetermined voltage VH (H level) corresponding to a logical value of “1” is output as an SQ sensor signal SSQ indicating a determination result that either adjacent interference or multipath interference has occurred. However, if VSQ≦Vref1, a predetermined voltage VL (L level, VL<VH) corresponding to a logical value of “0” is output as SSQ indicating a determination result that neither adjacent interference nor multipath interference has occurred. In an FM tuner supporting RDS, an AF search is performed for automatically selecting a broadcasting station with favorable reception conditions. For example, SSQ is utilized in assessing the reception state when automatic channel selection is performed.
A circuit composed of an HPF 18 and a detection circuit 20 may be provided as another circuit for detecting adjacent interference. This circuit outputs a DC signal SAI of a voltage level that corresponds to the strength of a high-frequency component that can be generated by adjacent interference. For example, the cut-off frequency fc of the HPF 18 may be approximately 100 kHz.
The VSQ or SAI corresponding to an adjacent interference component is used in a bandwidth control circuit 22 for controlling a bandwidth WF of the IFBPF 4. The bandwidth control circuit 22 narrows WF in instances in which adjacent interference has occurred, and reduces the effect of adjacent interference on the output audio signal. The detection circuits 14, 20 perform the detecting using CR-assisted smoothing, and the detection outputs VSQ, SAI accordingly provide a predetermined time constant. In a case where the VSQ or SAI is used to control of the bandwidth WF of the IFBPF 4, the time constants will have the effect of minimizing the effect on the output acoustic signal caused by WF being frequently switched between high and low.
A comparator 24, an SD band judgment circuit 26, and an SD circuit 28 are provided as circuits for detecting a broadcasting station when the aforedescribed AF search or another type of automatic station selection is performed. SD denotes “station detection”. The comparator 24 compares the reception field strength signal SM-DC received from the S meter circuit 10, and a reference voltage Vref2 set to a predetermined threshold value; and emits an H level output to the SD circuit 28 if SM-DC≧Vref2, or an L level output if SM-DC<Vref2.
The FM detection circuit 8 detects the output signal of the limiter amplifier 6, and generates SDET. SDET is fed to a stereo demodulating circuit (not shown). The stereo demodulating circuit demodulates the detection signal to an audio signal composed of an R channel signal and an L channel signal. The audio signal is fed to output terminals of a speaker or the like.
A f-V conversion (“S curve”) characteristic exists between a frequency f of the intermediate signal SIF2 received by the FM detection circuit 8 and a detection output voltage V. The SD band judgment circuit 26 uses the f-V conversion characteristic, and, based on a null voltage ΔV generated from the detection signal SDET produced by the FM detection circuit 8, determines whether the band of the reception station lies within the target band (SD band).
The null voltage ΔV is a signal that corresponds to the difference between an AFC voltage VAFC and a reference voltage Vref3, with VAFC being generated once the detection output SDET of the FM detection circuit 8 has been smoothed by a capacitor CAFC that is earthed on one terminal. A resistor RAFC that acts as a load on the VAFC connects the VAFC and the reference voltage Vref3, and the voltage generated between the terminals of the RAFC enters the SD band judgment circuit 26 as ΔV. Vref3 can be set so that the V at the intermediate frequency fIF2 will be such that ΔV=0. FIG. 5 is a graph that schematically depicts the f-V conversion characteristic, with the horizontal axis showing the frequency f, and the vertical axis showing the null voltage ΔV.
The SD band has the intermediate frequency at its center, and is set to a predetermined width to avoid being adversely affected by signals from adjacent channels. The SD band judgment circuit 26 is configured using a window comparator, and is set so that the window is a voltage range associated with the SD band in accordance with the f-V conversion characteristic. In a case where the null voltage ΔV lies within the window, the SD band judgment circuit 26 outputs an H-level SD band judgment signal to the SD circuit 28.
The SD circuit 28 outputs the logical product (AND) of the outputs of the comparator 24 and the SD band judgment circuit 26 as an SD signal SSD. The SD signal indicates whether the reception station has been detected at the tuning frequency set during automatic station selection. In a case where a reception signal of a predetermined strength is obtained in the SD band, the SD circuit 28 outputs an H level that shows that the reception station has been detected.
With RDS reception, an AF search is performed and a broadcasting station having a favorable reception state is automatically selected in order to receive an FM broadcast when the signal quality is consistently good, or in cases where reception is poor. In order to minimize periods of disrupted reception, the AF search is preferably performed at as high a speed as possible. However, quickly detecting the reception state is not the only important criterion when an AF search is performed; the accuracy with which the detection is performed must also be taken into consideration.
European RDS receivers in particular use only one channel in order to keep costs low. In order to perform an AF search using one channel, means is required for moving the reception channel to another broadcast channel for a limited period of time; and then accurately detecting adjacent interference while performing PLL locking, detecting stations, detecting adjacent interference, detecting multipath interference, and performing other processes.
However, problems have been presented in that adjacent interference is difficult to detect at a reliable degree of accuracy when the SQ sensor signal SSQ or the signal SAI generated from SDET is used in order to determine the presence of adjacent interference in the AF search. This is because the FM-modulated signals are superimposed on the AC component extracted from SM-AC and SDET by the HPF; and SSQ and SAI, which are produced therefrom, change according to the degree of FM modulation. A further problem is presented in that the AC component cannot be obtained from SM-AC or SDET in a case where the adjacent interference signal is greater than the desired wave signal, and the reception channel is completely captured by the adjacent interference signal.
The low reliability with which adjacent interference is accurately detected as described above makes it difficult not only to suitably control the pass bandwidth of the IFBPF 4 used to reduce adjacent interference, but to determine in an accurate manner whether or not adjacent interference is present in the channel currently being received. As a result, complications are presented in precisely ascertaining the incoming signal condition of the channel currently being received, which is an important consideration when AF searching is performed.